Kapsama ve Mutasyon — Yüzde Yüz Test, Hata Nasıl Geçti?

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public final class TransferFee {
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    private static final BigDecimal LARGE = new BigDecimal("10000");
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    private static final BigDecimal RATE = new BigDecimal("0.001");
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    private static final BigDecimal FLAT = new BigDecimal("5");
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    private TransferFee() {
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    }
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    public static BigDecimal of(BigDecimal amount) {
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        if (amount.signum() <= 0) {
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            throw new IllegalArgumentException("Tutar sıfırdan büyük olmalı.");
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        }
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        if (amount.compareTo(LARGE) >= 0) {
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            return amount.multiply(RATE).setScale(2, RoundingMode.HALF_EVEN);
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        }
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        return FLAT;
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    }
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}

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class TransferFeeTest {
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    @Test
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    void a_small_transfer_pays_the_flat_fee() {
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        assertThat(TransferFee.of(new BigDecimal("500"))).isEqualByComparingTo("5");
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    }
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    @Test
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    void a_large_transfer_pays_one_per_mille() {
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        assertThat(TransferFee.of(new BigDecimal("20000"))).isEqualByComparingTo("20");
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    }
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    @Test
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    void exactly_ten_thousand_already_counts_as_large() {
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        // The boundary: kills the ">= becomes >" mutant.
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        assertThat(TransferFee.of(new BigDecimal("10000"))).isEqualByComparingTo("10");
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    }
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    @Test
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    void zero_and_negative_amounts_are_rejected() {
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        assertThatThrownBy(() -> TransferFee.of(BigDecimal.ZERO)).isInstanceOf(IllegalArgumentException.class);
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        assertThatThrownBy(() -> TransferFee.of(new BigDecimal("-1"))).isInstanceOf(IllegalArgumentException.class);
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    }
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}

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<plugin>
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  <groupId>org.pitest</groupId>
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  <artifactId>pitest-maven</artifactId>
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  <configuration>
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    <targetClasses>
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      <param>com.bank.payments.*</param>
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    </targetClasses>
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    <!-- On pull requests, only mutate what changed. -->
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    <features>
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      <feature>+GIT(from[HEAD~1])</feature>
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    </features>
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    <mutationThreshold>80</mutationThreshold>
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  </configuration>
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</plugin>

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// Tempting, but look what happens: every line runs, nothing is checked.
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class TransferFeeHollowTest {
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    @Test
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    void runs_everything() {
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        TransferFee.of(new BigDecimal("500"));
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        TransferFee.of(new BigDecimal("20000"));
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        try {
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            TransferFee.of(new BigDecimal("-1"));
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        } catch (IllegalArgumentException ignored) {
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            // coverage: 100%, mutants killed: 0
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        }
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    }
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}